For the decontamination of exhaust gases from internal combustion engines, in addition to the oxidation catalysts also multifunctional catalysts, so-called three-way catalysts, are being used more and more. These three-way catalyst have the characteristic of simultaneously converting the harmful substances of CO, HC and NO.sub.X in a certain area of the exhaust gas composition. In order to maintain this area of the exhaust gas composition, the catalysts in motor vehicles are used in combination with a controllable carburetor or an injection device ("closed loop") for the cleaning of the exhaust gas. At the same time, the O.sub.2 content of the exhaust gas is determined by means of an oxygen sensor, the so-called .lambda. (lambda) - sensor. The sensor produces a voltage up to 800 mV, corresponding to the O.sub.2 partial pressure prevailing in the exhaust gas. This voltage is electronically amplified and controls via a regulator the formation of the mixture of the injection system or of a carburetor. The control should take place in such a way that the composition of the exhaust gas mixture always makes possible the optimum and simultaneous conversion of CO, HC and NO.sub.X in as narrow band width as possible at the most favorable point of operation of the catalyst.
The mean operating range of currently used three-way catalysts lies at .lambda.=0.985 -1.00; that is to say, barely in the rich area of the air/fuel mixture. The oxygen sensor consequently is admitted with rich exhaust gas. As has been found, the sensor signal, which is measured before the catalyst, is adulterated by the hydrogen and CO components contained in the exhaust gas. This sensor signal is then not well defined. As a result of this, the sensor voltage exceeds the control range of the system which leads to an overshooting of the mixture formation and thus to an increase of the emissions. Especially in the case of converting the nitric oxides however a precise adjustment of the formation of the mixture has a favorable effect on the optimum operation point of a three-way catalyst.